This invention relates generally to ion filters such as used in mass spectrometers and mass analyzers, and more particularly the invention relates to arcuate (including circular segments) hyperbolic quadrapoles.
Mass spectrometers are well known scientific instruments for analyzing chemical structures. A mass spectrometer includes an ion source, an ion filter, and an ion detector. Gas at low pressure is introduced into the ion source which ionizes the gas. Ions are then selected by the ion filter and passed to the ion detector. The ion filter selects ions having a particular m/e ratio which may be varied to analyze the gas. Examples of mass spectrometers are described in U.S. Pat. Nos. 5,389,785; 5,298,745; 4,949,047; 4,885,470; 4,158,771; and 3,757,115.
These and other known mass spectrometers employ filters having linear hyperbolic quadrapoles. FIG. 1 is a schematic perspective view of the ion filter described in U.S. Pat. No. 5,389,785 to Urs Steiner et al. The filter includes four linear rod electrodes 3, each rod electrode having a hyperbolic curved pole surface 5 and two abutment surfaces 7 so that the rods can be assembled with opposing pairs of hyperbolic surfaces. Rods with opposing hyperbolic surfaces are electrically connected, and both RF and DC voltages are impressed on the rods with the RF voltages on adjacent poles being out of phase and the DC voltages on adjacent poles being offset.
The hyperbolic surfaces must be very smooth and accurately machined. Due to the linear configuration of the pole segments, a grinding tool is used in machining the surfaces. This necessarily limits the accuracy in machining the surfaces.